1. Field of the Invention
The present invention relates to a liquid crystal display device which is used, for example, as a vehicle meter for indicating a speed or number of revolutions.
2. Description of the Related Art
A liquid crystal display device (hereinafter referred to as "LCD" device for brevity), which should be used as a vehicle display device, must operate at a temperature ranging from -30.degree. C. to 85.degree. C. taking an environment around a vehicle into consideration. However, regarding the temperature characteristic of liquid crystal, the response of the liquid crystal is very poor at a low temperature. FIG. 7 is a characteristic graph showing a relationship between a temperature and the response speed of an LCD element. As seen from the graph, with respect to the response speed (or time) of the LCD element, both the rising time tr and dropping time td thereof increase at a low temperature. FIGS. 8A to 8C are response waveforms showing the response characteristic of the LCD element at room temperature and a low temperature. FIG. 8A shows an LCD driving waveform; FIG. 8B shows a response waveform of the LCD element at room temperature; and FIG. 8C shows a response waveform of the LCD element at a low temperature. In FIGS. 8A to 8C, time interval tch1 denotes an indication updating time in a 7-segment display having a ".theta." shape capable of indicating digits of 0-9 in which "on" and "off" of each segment are synchronized with one another to update the indication for each time interval.
As seen from FIGS. 8A to 8C, at room temperature, the sum of the rising time tr and dropping time td is shorter than the indication updating time tch1 of the LCD driving waveform (FIG. 8A). On the other hand, at a lower temperature, the former is longer than the latter (FIG. 8C). Therefore, when the driving waveform is shifted from "OFF" to "ON", it becomes "OFF" before the light transmissivity of the LCD element reaches 90%. Also, when the driving waveform is shifted from "ON" to "OFF", it becomes "ON" before the light transmissivity of the LCD element reaches 10%. Thus, at the low temperature, on the moving picture of a speed meter, rotation meter, television, etc., in which the indication contents change at a high speed, a previous image and a subsequent image are mixedly displayed, thereby making an indication disabling state.
An exemplary means for solving such a problem is disclosed in Japanese U. M. 56-128618. In this prior art, the temperature of an LCD element, which is detected by a temperature detecting circuit, is forcibly elevated by a heater such as a heating wire at a low temperature so that it is early elevated to a normal operating temperature.
The prior art, which is constructed as described above, requires a heater such a heating wire and hence has problems of an increase in cost and weight and limitation of layout due to large-scaling.